Power supply circuits are widely used in modern electronic products such as flat panel displays. The power supply circuit is typically used for switching between two or more input voltage signals when the electronic product is in different working states.
FIG. 2 is a circuit diagram of a conventional power supply circuit used in a liquid crystal display (LCD). The power supply circuit 10 includes a scaler 11, a soft start circuit 13, a pulse width modulation circuit 15, an inverter 17, and a start-up capacitor 19.
The scaler 11 is connected to the pulse width modulation circuit 15 via the soft start circuit 13. Therefore, the soft start circuit 13 functions as a switch element for switching the pulse width modulation circuit 15. The pulse width modulation circuit 15 controls the inverter 17 to output driving voltages, for driving an electronic apparatus (not shown).
However, the soft start circuit 13 includes two bipolar junction transistors 131, 133 and three bias resistors 135, 137, 139. The soft start circuit 13 has a complex circuit configuration. Moreover, the bipolar junction transistors 131, 133 are current-controlled switch elements. Thus, a direct current (DC) power supply 102 is needed to supply bias voltage to the soft start circuit 13. Thus, the power supply circuit 10 has high power consumption. Meanwhile, the complex circuit configuration of the soft start circuit 13 is liable to causes some electric noise. The electric noise causes negative influence to a reliability of the power supply circuit 10.
Accordingly, what is needed is a power supply circuit that can overcome the above-described deficiencies.